Communication method between IPv4 terminal and IPv6 terminal and IPv4-IPv6 converting apparatus

This is a continuation application of U.S. Ser. No. 09/614,715, filed Jul. 12, 2000 now U.S. Pat. No. 6,690,669, which is a continuation application of U.S. Ser. No. 08/959,462, filed Oct. 28, 1997, now U.S. Pat. No. 6,118,784.

BACKGROUND OF THE INVENTION

The present invention relates to a communicating method between an IPv4 terminal and an IPv6 terminal and to an IPv4-IPv6 converting apparatus. More particularly, the invention relates to method and apparatus for realizing a communication between an IPv4 terminal using IPv4 (Internet Protocol version 4) as a communication protocol and an IPv6 terminal using IPv6 (Internet Protocol version 6) as a communication protocol.

As a method of realizing a communication between an IPv4 terminal and an IPv6 terminal, there is a method of using an ‘IPv4-mapped IPv6 address’ and an ‘IPv4-compatible IPv6 address’ described in RFC (Request For Comments) 1884 issued by IETF (Internet Engineering Task Force).

The ‘IPv4-mapped IPv6 address’ is an IPv6 address in which “0” is set to the 127th to 48th bits among 128 bits of the IPv6 address, “1” is set to the 47th to 32nd bits, and an ‘IPv4 address’ is set to the 31st to 0th bits.

The ‘IPv4-compatible IPv6 address’ is an IPv6 address in which “0” is set to the 127th to 32nd bits among 128 bits of the IPv6 address and the ‘IPv4 address’ is set to the 31st to 0th bits.

When the communication between the IPv4 terminal and the IPv6 terminal is executed, an IPv4 address is preliminarily fixedly allocated to the IPv6 terminal which communicates with the IPv4 terminal. A packet converting apparatus is provided on the way of a path through which the IPv4 terminal and the IPv6 terminal communicate and a mutual conversion of IP headers of an IPv4 packet (RFC791) and an IPv6 packet (RFC1883) is performed.

In an IPv4 network, the IPv4 address of the IPv4 terminal and the IPv4 address allocated to the IPv6 terminal are used and a communication is executed by using the IPv4 packet. In an IPv6 network, the ‘IPv4-mapped IPv6 address’ in which the ‘IPv4 address of the IPv4 terminal’ has been set is used as an IPv6 address of the IPv4 terminal. In the IPv6 network, the ‘IPv4-compatible IPv6 address’ in which the ‘IPv4 address allocated to the IPv6 terminal’ has been set is used as an IP address of the IPv6 terminal and a communication is executed by using the IPv6 packet.

The packet converting apparatus converts the ‘IPv4-mapped IPv6 address’ included in the IPv6 packet into the ‘IPv4 address of the IPv4 terminal’ and converts the ‘IPv4-compatible IPv6 address’ to the ‘IPv4 address allocated to the IPv6 terminal’, thereby converting the IPv6 packet into the IPv4 packet. Contrarily, the ‘IPv4 address of the IPv4 terminal’ included in the IPv4 packet is converted into the ‘IPv4-mapped IPv6 address’ and the ‘IPv4 address allocated to the IPv6 terminal’ is converted into the ‘IPv4-compatible IPv6 address’, thereby converting the IPv4 packet into the IPv6 packet. Consequently, the communication between the IPv4 terminal and the IPv6 terminal can be executed.

As a method of realizing the communication between the IPv4 terminal and the IPv6 terminal, IETF also proposed a method called a dual stack. According to the method, the IPv6 terminal has both of the communication protocols of IPv4 and IPv6. A communication is executed between the IPv6 terminals by using the IPv6 protocol. A communication is executed between the IPv4 terminals by using the IPv4 protocol. Thus, the communication between the IPv4 terminal and the IPv6 terminal can be executed.

IETF also proposed a method called an IP tunneling. This is a method such that when the IPv4 network exists on a communication path between two IPv6 terminals and a communication cannot be executed directly by the IPv6 packet, the IPv6 packet is encapsulated by the IPv4 header and is allowed to pass through the IPv4 network. Similarly, when the IPv6 network exists on a communication path between the IPv4 terminals and a communication cannot be executed directly by the IPv4 packet, the IPv4 packet is encapsulated by the IPv6 header and is allowed to pass through the IPv6 network. Consequently, even when there is the IPv4 network on the communication path, the communication between the IPv6 terminals can be executed. Even when there is the IPv6 network on the communication path, the communication between the IPv4 terminals can be executed.

According to the method, the communication between the IPv4 terminal and the IPv6 terminal can be realized by a simple operation of adding a fixed pattern of 96 bits to the IPv4 address or deleting the fixed pattern of 96 bits from the IPv6 address.

According to the method called a dual stack, by selectively using the communication protocols of IPv4 and IPv6 in accordance with a communication partner, the communication between the IPv4 terminal and the IPv6 terminal can be realized.

According to the method called an IP tunneling, by encapsulating the packet by the header of the relevant communication protocol and passing the resultant data through the network existing on the communication path between the two terminals, the communication between the two terminals can be realized.

SUMMARY OF THE INVENTION

The convention techniques, however, have the following problems.

A first problem is that the IPv4 address has to be fixedly allocated to all of the IPv6 terminals which have a possibility of communication with the IPv4 terminal. This further consumes the IPv4 address which is short and accelerates its shortages.

A second problem is that since mutual domain information (for example, an IP address corresponding to a domain name) cannot be referred to between the IPv4 network and the IPv6 network, the partner can be designated not by the domain name but by only an actual IP address. This results in that the communication partners have to be mutually aware whether the partner uses IPv4 or IPv6 as a communication protocol.

A third problem is that, in the method called a dual stack, the IPv4 protocol is provided for all of the IPv6 terminals having the possibility of communication with the IPv4 terminal and the IPv4 address has to be fixedly allocated. This results in that a larger memory capacity is necessary as compared with that in case of providing only the IPv6 protocol and, in a manner similar to the first problem, the IPv4 address which is short is further consumed and the shortage is accelerated.

A fourth problem is that, in the method called an IP tunneling, when the IPv4 network exists on the communication path between the two IPv6 terminals, in order to pass the IPv6 packet by encapsulating it by the IPv4 header, a setting for it has to be preliminarily performed. Similarly, when the IPv6 network exists on the communication path between the two IPv4 terminals, in order to pass the IPv4 packet by encapsulating it by the IPv6 header, a setting for it has to be preliminarily performed. The method can be used only between the terminals in which the setting of the IP tunneling has preliminarily been performed. The communication cannot be executed between the terminals in which the setting is not preliminarily performed.

It is, therefore, an object of the invention to provide method and apparatus in which only an IPv6 protocol is provided for an IPv6 terminal, even if an IPv4 address is not preliminarily fixedly allocated, a communication between an IPv4 terminal and the IPv6 terminal can be executed, a partner is designated not by an IP address but by a domain name, and the communication between the IPv4 terminal and the IPv6 terminal can be executed.

According to the first aspect, in a communication network system comprising an IPv4 network, an IPv6 network; and an IPv4-IPv6 converting apparatus connected to both of them, an IPv4 terminal of the IPv4 network inquires an IP address for a domain name of an IPv6 terminal of the IPv6 network to the IPv4-IPv6 converting apparatus. Thus, the IPv4-IPv6 converting apparatus obtains the IPv6 address corresponding to the domain name from a DNS (Domain Name System) v6 server apparatus for managing domain information of the IPv6 network, dynamically acquires the IPv4 address corresponding to the IPv6 address from a DHCP (Dynamic Host Configuration Protocol) v4 server apparatus for dynamically allocating the IPv4 address, and notifies to the IPv4 terminal. The IPv4 terminal forms an IPv4 packet by setting an own IPv4 address to an IPv4 source address and by setting the notified IPv4 address to an IPv4 destination address and sends the IPv4 packet to the IPv4-IPv6 converting apparatus. The IPv4-IPv6 converting apparatus converts the IPv4 source address included in the received IPv4 packet into an IPv6 source address by adding fixed data to the IPv4 source address and converts an IPv4 destination address included in the IPv4 packet into the IPv6 address corresponding to the IPv4 destination address, thereby obtaining an IPv6 destination address, forming an IPv6 packet from the IPv4 packet, and sending the IPv6 packet to the IPv6 network. The IPv6 terminal forms the IPv6 packet by setting the own IPv6 address to the IPv6 source address and setting the IPv6 source address included in the received IPv6 packet to the IPv6 destination address, and transmits the IPv6 packet to the IPv4-IPv6 converting apparatus. The IPv4-IPv6 converting apparatus deletes the fixed data from the IPv6 destination address included in the received IPv6 packet, converts the resultant IPv6 destination address to the IPv4 destination address, and converts the IPv6 source address included in the IPv6 packet to the IPv4 address corresponding to the IPv6 source address, thereby obtaining the IPv4 source address, forming the IPv4 packet from the IPv6 packet, and transmitting the IPv4 packet to the IPv4 network.

In the communicating method between the IPv4 terminal and the IPv6 terminal according to the first aspect, when the communication from the IPv4 terminal side of the IPv4 network to the IPv6 terminal of the IPv6 network is started, the IPv4 address is dynamically allocated to the IPv6 terminal of the IPv6 network. Consequently, it is unnecessary to preliminarily fixedly allocate the IPv4 address to the IPv6 terminal, so that the consumption of the IPv4 address can be suppressed. Since the IPv4-IPv6 converting apparatus automatically obtains the IP address for the domain name of the IPv6 terminal from the DNSv6 server apparatus, the IPv4 terminal can designate the domain name and communicate with the IPv6 terminal.

According to the second aspect, in a communication network system comprising an IPv4 network, an IPv6 network, and an IPv4-IPv6 converting apparatus connected to both of them, an IPv6 terminal of the IPv6 network inquires an IP address for a domain name of an IPv4 terminal of the IPv4 network to the IPv4-IPv6 converting apparatus. Thus, the IPv4-IPv6 converting apparatus acquires an IPv4 address corresponding to the domain name from a DNSv4 server apparatus for managing domain information of the IPv4 network, converts the IPv4 address to the IPv6 address by adding fixed data to the IPv4 address, and notifies the IPv6 terminal of the IPv6 address. The IPv6 terminal forms an IPv6 packet by setting an own IPv6 address to an IPv6 source address and setting the notified IPv6 address to an IPv6 destination address and transmits the IPv6 packet to the IPv4-IPv6 converting apparatus. The IPv4-IPv6 converting apparatus deletes the fixed data from the IPv6 destination address included in the received IPv6 packet and converts the resultant IPv6 destination address to an IPv4 destination address, dynamically acquires the IPv4 address corresponding to the IPv6 source address included in the IPv6 packet from a DHCPv4 server apparatus for dynamically allocating the IPv4 address, forms the IPv4 packet from the IPv6 packet by using the IPv4 address, and transmits the IPv4 packet to the IPv4 network. The IPv4 terminal forms the IPv4 packet by setting the own IPv4 address to an IPv4 source address and setting the IPv4 source address included in the received IPv4 packet to an IPv4 destination address and transmits the IPv4 packet to the IPv4-IPv6 converting apparatus. The IPv4-IPv6 converting apparatus converts the IPv4 source address to the IPv6 source address by adding fixed data to the IPv4 source address, obtains the IPv6 destination address by converting the IPv4 destination address included in the IPv4 packet to the IPv6 address corresponding to the IPv4 destination address, forms the IPv6 packet from the IPv4 packet, and transmits the IPv6 packet to the IPv6 network.

In the communicating method between the IPv4 terminal and the IPv6 terminal according to the second aspect, when the communication from the IPv6 terminal side of the IPv6 network to the IPv4 terminal of the IPv4 network is started, the IPv4 address is dynamically allocated to the IPv6 terminal of the IPv6 network. Consequently, it is unnecessary to preliminarily fixedly allocate the IPv4 address to the IPv6 terminal, so that the consumption of the IPv4 address can be suppressed. Since the IPv4-IPv6 converting apparatus automatically obtains the IP address for the domain name of the IPv4 terminal from the DNSv4 server apparatus, the IPv6 terminal can designate the domain name and communicate with the IPv4 terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus according to the first embodiment of the invention;

<figref idref="DRAWINGS">FIG. 2</figref> is a constructional diagram of an IP address conversion table;

<figref idref="DRAWINGS">FIG. 3</figref> is a constructional diagram of a communication network system according to the first embodiment of the invention;

<figref idref="DRAWINGS">FIG. 4</figref> is a flowchart in case of starting a communication from an IPv4 terminal to an IPv6 terminal through the IPv4-IPv6 converting apparatus according to the first embodiment of the invention;

<figref idref="DRAWINGS">FIG. 5</figref> is a flowchart which is continuous to <figref idref="DRAWINGS">FIG. 4</figref>;

<figref idref="DRAWINGS">FIG. 6</figref> is a flowchart in case of starting a communication from the IPv6 terminal to the IPv4 terminal through the IPv4-IPv6 converting apparatus according to the first embodiment of the invention;

<figref idref="DRAWINGS">FIG. 7</figref> is a flowchart which is continuous to <figref idref="DRAWINGS">FIG. 6</figref>;

<figref idref="DRAWINGS">FIG. 8</figref> is a flowchart in another case of starting a communication from the IPv6 terminal to the IPv4 terminal;

<figref idref="DRAWINGS">FIG. 9</figref> is a flowchart which is continuous to <figref idref="DRAWINGS">FIG. 8</figref>;

<figref idref="DRAWINGS">FIG. 10</figref> is a constructional diagram of another IPv4-IPv6 converting apparatus;

<figref idref="DRAWINGS">FIG. 11</figref> is a constructional diagram of another network system;

<figref idref="DRAWINGS">FIG. 12</figref> is a flowchart in another case of starting a communication from the IPv4 terminal to the IPv6 terminal;

<figref idref="DRAWINGS">FIG. 13</figref> is a flowchart which is continuous to <figref idref="DRAWINGS">FIG. 12</figref>;

<figref idref="DRAWINGS">FIG. 14</figref> is a flowchart in still another case of starting a communication from the IPv6 terminal to the IPv4 terminal;

<figref idref="DRAWINGS">FIG. 15</figref> is a flowchart which is continuous to <figref idref="DRAWINGS">FIG. 14</figref>;

<figref idref="DRAWINGS">FIG. 16</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus according to the second embodiment of the invention;

<figref idref="DRAWINGS">FIG. 17</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus according to the third embodiment of the invention;

<figref idref="DRAWINGS">FIG. 18</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus according to the fourth embodiment of the invention;

<figref idref="DRAWINGS">FIG. 19</figref> is a constructional diagram of a communication network system according to the fifth embodiment of the invention;

<figref idref="DRAWINGS">FIG. 20</figref> is a flowchart in case of communicating between the IPv6 terminals through the IPv4-IPv6 converting apparatus according to the fifth embodiment of the invention; and

<figref idref="DRAWINGS">FIG. 21</figref> is a flowchart which is continuous to FIG. <b>20</b>.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described hereinbelow with reference to the drawings.

(A) First Embodiment

<figref idref="DRAWINGS">FIG. 1</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus <b>1</b><i>a </i>according to the first embodiment of the invention.

The IPv4-IPv6 converting apparatus <b>1</b><i>a </i>is located between an IPv4 network and an IPv6 network and realizes a communication between an IPv4 terminal and an IPv6 terminal by performing an IP header conversion of an IPv4 packet and an IPv6 packet. The IPv4-IPv6 converting apparatus <b>1</b><i>a </i>comprises IP transmitting and receiving means <b>10</b>, IP header converting means <b>11</b>, DNS substituting means <b>13</b>, IPv4 address capturing means <b>14</b>, and IP address conversion information holding means <b>15</b>.

The IP transmitting and receiving means <b>10</b> is means for transmitting and receiving the IPv4 packet and IPv6 packet between the IPv4 network and the IPv6 network and is constructed by, for example, electronic devices such as CPU, RAM, and the like.

The IP header converting means <b>11</b> is means for performing an IP header conversion of the IPv4 packet and the IPv6 packet and is constructed by, for example, electronic devices such as CPU, RAM, and the like.

The DNS substituting means <b>13</b> is means for capturing domain information from a DNS server in the IPv4 network or a DNS server in the IPv6 network in accordance with the DNS technique disclosed in RFC1034, RFC1886, or the like and is constructed by, for example, electronic devices such as CPU, RAM, and the like.

The IPv4 address capturing means <b>14</b> is means for capturing an IPv4 address from a DHCP server in the IPv4 network in accordance with the DHCP technique disclosed in RFC1541 or the like and is constructed by, for example, electronic devices such as CPU, RAM, and the like.

The IP address conversion information holding means <b>15</b> is means for converting the IPv4 address and the IPv6 address and is constructed by, for example, electronic devices such as an RAM and the like. A correspondence between the IPv4 address and the IPv6 address is held as an IP address conversion table <b>151</b>.

<figref idref="DRAWINGS">FIG. 2</figref> is a constructional diagram of the IP address conversion table <b>151</b>. IPv6 addresses of the IPv6 terminal communicating with the IPv4 terminal and IPv4 addresses which are dynamically allocated in correspondence to them have been registered in the IP address conversion table <b>151</b>.

<figref idref="DRAWINGS">FIG. 3</figref> is a diagram of a communication network system <b>100</b> connecting an IPv4 network <b>104</b> and an IPv6 network <b>106</b> through the IPv4-IPv6 converting apparatus <b>1</b><i>a. </i>

In addition to an IPv4 terminal <b>2</b>, a DNSv4 server <b>3</b> for managing domain information in the IPv4 network <b>104</b> and a DHCPv4 server <b>4</b> for dynamically allocating the IPv4 addresses to terminals in the IPv4 network <b>104</b> are connected to the IPv4 network <b>104</b>.

In addition to an IPv6 terminal <b>5</b>, a DNSv6 server <b>6</b> for managing domain information in the IPv6 network is connected to the IPv6 network <b>106</b>.

It is now assumed that “<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>” has been allocated as an IPv4 address to the IPv4 terminal <b>2</b> and “<b>1</b> : : <b>1</b>” has been allocated as an IPv6 address to the IPv6 terminal <b>5</b>. It is also assumed that 100 IPv4 addresses of “<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>” to “<b>133</b>. <b>144</b>. <b>95</b>. <b>100</b>” have been pooled in the DHCPv4 server <b>4</b>. It is also assumed that the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>has been set as a DNS server into the IPv4 terminal <b>2</b> and IPv6 terminal <b>5</b>. Further, it is assumed that the DNSv4 server <b>3</b> and DNSv6 server <b>6</b> have been set as DNS servers and the DHCPv4 server <b>4</b> has been set as a DHCP server into the IPv4-IPv6 converting apparatus <b>1</b><i>a. </i>

In <figref idref="DRAWINGS">FIG. 3</figref>, although the IPv4 network <b>104</b> and IPv6 network <b>106</b> are perfectly separated, a similar construction is also applied to a case where the IPv4 network and IPv6 network physically mixedly exist in the same network.

<figref idref="DRAWINGS">FIGS. 4 and 5</figref> are flowcharts of the operation in case of starting a communication from the IPv4 terminal <b>2</b> to the IPv6 terminal <b>5</b>.

Although the IPv4 terminal <b>2</b> knows a domain name of the IPv6 terminal <b>5</b>, it doesn't know an IP address. As shown in <figref idref="DRAWINGS">FIG. 4</figref>, therefore, after the IPv4 terminal <b>2</b> transmitted an inquiry message (hereinafter, called a ‘message A’) of the IP address corresponding to the domain name of the IPv6 terminal <b>5</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>via the IPv4 network <b>104</b>, it waits for a response for a predetermined time.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>receives and processes the message A and sends it to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message A to the DNSv4 server <b>3</b>, sends the message A thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message A to the DNSv4 server <b>3</b> via the IPv4 network.

The DNSv4 server <b>3</b> which received the message A retrieves the domain information of the IPv6 terminal <b>5</b>. When it is not registered, however, a response to the message A is not returned.

The DNS substituting means <b>13</b> which didn't receive the response from the DNSv4 server <b>3</b> even after the elapse of a predetermined time instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message A to the DNSv6 server <b>6</b>, sends the message A thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message A to the DNSv6 server <b>6</b> via the IPv6 network.

The DNSv6 server <b>6</b> which received the message A retrieves the domain information of the IPv6 terminal <b>5</b> and returns the registered domain information of the IPv6 terminal <b>5</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>as a response message (hereinafter, referred to as a ‘message B’) for the message A.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>which received the response from the DNSv6 server <b>6</b> within the predetermined time sends the received message B to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> obtains the IPv6 address (<b>1</b> : : <b>1</b>) corresponding to the domain name of the IPv6 terminal <b>5</b> from the message B and sends it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> as a key. If the relevant entry has been registered, the IPv4 address of the entry is returned to the DNS substituting means <b>13</b>. On the other hand, when the relevant entry is not registered yet, such a fact is notified to the DNS substituting means <b>13</b>.

The DNS substituting means <b>13</b> which received the IPv4 address from the IP address conversion information holding means <b>15</b> forms a message (hereinafter, referred to as a ‘message E’) to notify of the IPv4 address as an IPv4 address corresponding to the domain name of the IPv6 terminal <b>5</b> and sends it to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the message E to the IPv4 terminal <b>2</b> via the IPv4 network.

On the other hand, the DNS substituting means <b>13</b> which received the notification indicating that the entry is not registered yet from the IP address conversion information holding means <b>15</b> instructs the IPv4 address capturing means <b>14</b> so as to capture the IPv4 address and, after that, waits for a response for a predetermined time. The IPv4 address capturing means <b>14</b> forms an IPv4 address capturing message (hereinafter, called a ‘message C’), instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message C to the DHCPv4 server <b>4</b>, sends the message C thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message C to the DHCPv4 server <b>4</b> via the IPv4 network.

The DHCPv4 server <b>4</b> which received the message C gives the IPv4 address to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> (it is now assumed that “<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>” has been given) and returns an IPv4 address given message (hereinafter, called a ‘message D’).

The IP transmitting and receiving means <b>10</b> which received the message D sends the message D to the IPv4 address capturing means <b>14</b>. The IPv4 address capturing means <b>14</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the message D and notifies the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> forms the message E to notify of the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) as an IP address corresponding to the domain name of the IPv6 terminal <b>5</b> and sends it to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the message E to the IPv4 terminal <b>2</b> via the IPv4 network. The DNS substituting means <b>13</b> sends the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) captured from the DHCPv4 server <b>4</b> to the IP address conversion information holding means <b>15</b> in correspondence to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers a correspondence of the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) and the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The entry of the IP address conversion table <b>151</b> is deleted in the case where it is not referred for a predetermined time. The IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) is returned to the DHCPv4 server <b>4</b>.

As shown in <figref idref="DRAWINGS">FIG. 5</figref>, the IPv4 terminal <b>2</b> transmits the IPv4 packet in which the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) notified from the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>by the message E has been set into an IPv4 destination address field of the IPv4 packet and an own IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) has been set into an IPv4 source address field to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>through the IPv4 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>sends the received IPv4 packet to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> extracts the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) from the IPv4 source address field of the IPv4 packet, converts the IPv4 address into an ‘IPv4-mapped IPv6 address’ (: : FFFF : <b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of 128 bits by adding a fixed pattern of 96 bits mentioned above to the IPv4 address, and sets it into an IPv6 source address field of the IPv6 packet. The IP header converting means <b>11</b> extracts the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the IPv4 destination address field of the IPv4 packet and sends it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> obtains the IPv6 address (<b>1</b> : : <b>1</b>) corresponding to the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) with reference to the IP address conversion table <b>151</b> and sends it to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> sets the IPv6 address (<b>1</b> : : <b>1</b>) into an IPv6 destination address field of the IPv6 packet. The IPv6 packet formed by performing the IP header conversion as mentioned above is sent to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the IPv6 packet to the IPv6 terminal <b>5</b> through the IPv6 network.

In this manner, the communication from the IPv4 terminal <b>2</b> to the IPv6 terminal <b>5</b> can be started.

<figref idref="DRAWINGS">FIGS. 6 and 7</figref> are flowcharts for the operation in case of starting a communication from the IPv6 terminal <b>5</b> to the IPv4 terminal <b>2</b>.

Although the IPv6 terminal <b>5</b> knows a domain name of the IPv4 terminal <b>2</b>, it doesn't know the IP address. As shown in <figref idref="DRAWINGS">FIG. 6</figref>, therefore, the IPv6 terminal <b>5</b> transmits an inquiry message (hereinafter, called a ‘message K’) of an IP address corresponding to the domain name of the IPv4 terminal <b>2</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>via the IPv6 network <b>106</b> and, after that, waits for a response for a predetermined time.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>receives and processes the message K and sends it to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message K to the DNSv6 server <b>6</b>, sends the message K thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message K to the DNSv6 server <b>6</b> via the IPv6 network.

The DNSv6 server <b>6</b> which received the message K retrieves the domain information of the IPv4 terminal <b>2</b>. However, when it is not registered, a response to the message K is not returned.

The DNS substituting means <b>13</b> which didn't receive the response from the DNSv6 server <b>6</b> even after the elapse of a predetermined time instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message K to the DNSv4 server <b>3</b>, sends the message K thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message K to the DNSv4 server <b>3</b> via the IPv4 network.

The DNSv4 server <b>3</b> which received the message K retrieves the domain information of the IPv4 terminal <b>2</b> and returns the registered domain information of the IPv4 terminal <b>2</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>as a response message (hereinafter, called a ‘message L’) for the message K.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>which received the response from the DNSv4 server <b>3</b> within the predetermined time sends the received message L to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) corresponding to the domain name of the IPv4 terminal <b>2</b> from the message L.

Subsequently, the DNS substituting means <b>13</b> sends the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> as a key and notifies the DNS substituting means <b>13</b> of information about whether the relevant entry has been registered or not.

The DNS substituting means <b>13</b> which received a notification indicating that the relevant entry is not registered yet in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> instructs the IPv4 address capturing means <b>14</b> so as to capture the IPv4 address and, after that, waits for a response for a predetermined time. The IPv4 address capturing means <b>14</b> forms an IPv4 address capture message (hereinafter, called a ‘message M’), instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message M to the DHCPv4 server <b>4</b>, sends the message M thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message M to the DHCPv4 server <b>4</b> via the IPv4 network.

The DHCPv4 server <b>4</b> which received the message M gives the IPv4 address to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> (it is now assumed that “<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>” is given) and returns an IPv4 address given message (hereinafter, called a ‘message N’).

The IP transmitting and receiving means <b>10</b> which received the message N sends the message N to the IPv4 address capturing means <b>14</b>. The IPv4 address capturing means <b>14</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the message N and notifies the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> sends the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) captured from the DHCPv4 server <b>4</b> to the IP address conversion information holding means <b>15</b> in correspondence to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers a correspondence of the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) and the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The entry in the IP address conversion table <b>151</b> is deleted when it is not referred to for a predetermined time and the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) is returned to the DHCPv4 server <b>4</b>.

When a notification indicating that the relevant entry has been registered in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is received or when the registration of the relevant entry into the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is completed, the DNS substituting means <b>13</b> forms a message (hereinafter, called a ‘message P’) to notify of an ‘IPv4-mapped IPv6 address’ (: : FFFF : <b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) in which the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> has been set as an IP address corresponding to the domain name of the IPv4 terminal <b>2</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message P to the IPv6 terminal <b>5</b> and sends the message P thereto. The IP transmitting and receiving means <b>10</b> transmits the message P to the IPv6 terminal <b>5</b> via the IPv6 network.

As shown in <figref idref="DRAWINGS">FIG. 7</figref>, the IPv6 terminal <b>5</b> transmits the IPv6 packet in which the IPv6 address (: : FFFF : <b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) notified from the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>has been set into the IPv6 destination address field of the IPv6 packet and the own IPv6 address (<b>1</b> : : <b>1</b>) has been set into the IPv6 source address field to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>through the IPv6 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>sends the received IPv6 packet to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> extracts the IPv6 address (: : FFFF : <b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) from the IPv6 destination address field of the IPv6 packet, converts the IPv6 address into the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of <b>32</b> bits by deleting the fixed pattern of 96 bits mentioned above, and sets the IPv4 address into the IPv4 destination address field of the IPv4 packet. The IP header converting means <b>11</b> extracts the IPv6 address (<b>1</b> : : <b>1</b>) from the IPv6 source address field of the IPv6 packet and sends it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) corresponding to the IPv6 address (<b>1</b> : : <b>1</b>) with reference to the IP address conversion table <b>151</b> and sends it to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> sets the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) into the IPv4 source address field of the IPv4 packet. The IPv4 packet formed by performing the IP header conversion as mentioned above is sent to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the IPv4 packet to the IPv4 terminal <b>2</b> through the IPv4 network.

In this manner, the communication from the IPv6 terminal <b>5</b> to the IPv4 terminal <b>2</b> can be started.

Although the case where the IPv4 domain information had been registered in the DNSv4 server <b>3</b> has been described above, the IPv4 domain information can be also registered in the DNSv6 server <b>6</b>. In this case, the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>captures the IPv4 domain information by communicating with the DNSv6 server <b>6</b> by using the IPv6 protocol. Similarly, the IPv6 domain information can be also registered in the DNSv4 server <b>3</b>. In this case, the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>captures the IPv6 domain information by communicating with the DNSv4 server <b>3</b> by using the IPv4 protocol.

Explanation has been made with respect to the case where the address conversion is performed solely in the address field by adding or deleting the fixed pattern of 96 bits mentioned above or by referring to the IP address conversion table <b>151</b>. As for the address on the terminal side which started the communication, however, a combination of the address on the terminal side which started the communication and the port number of a TCP (Transmission Control Protocol)/UDP (User Datagram Protocol) can be also converted to a combination of an address of the IPv4-IPv6 converting apparatus and a port number of the TCP/UDP by using a method called an IP masquerade.

<figref idref="DRAWINGS">FIGS. 8 and 9</figref> are flowcharts for operation in another case of starting a communication from the IPv6 terminal <b>5</b> to the IPv4 terminal <b>2</b>. This case has a feature in that temporary allocation of IPv4 address to the IPv6 terminal is performed at the time of actually starting a data communication.

Although the IPv6 terminal <b>5</b> knows a domain name of the IPv4 terminal <b>2</b>, it doesn't know the IP address. As shown in <figref idref="DRAWINGS">FIG. 8</figref>, therefore, the IPv6 terminal <b>5</b> transmits an inquiry message (hereinafter, called a ‘message K’) of an IP address corresponding to the domain name of the IPv4 terminal <b>2</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>via the IPv6 network <b>106</b> and, after that, waits for a response for a predetermined time.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>receives and processes the message K and sends it to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message K to the DNSv6 server <b>6</b>, sends the message K thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message K to the DNSv6 server <b>6</b> via the IPv6 network.

The DNSv6 server <b>6</b> which received the message K retrieves the domain information of the IPv4 terminal <b>2</b>. However, when it is not registered, a response to the message K is not returned.

The DNS substituting means <b>13</b> which didn't receive the response from the DNSv6 server <b>6</b> even after the elapse of a predetermined time instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message K to the DNSv4 server <b>3</b>, sends the message K thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message K to the DNSv4 server <b>3</b> via the IPv4 network.

The DNSv4 server <b>3</b> which received the message K retrieves the domain information of the IPv4 terminal <b>2</b> and returns the registered domain information of the IPv4 terminal <b>2</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>as a response message (hereinafter, called a ‘message L’) for the message K.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>which received the response from the DNSv4 server <b>3</b> within the predetermined time sends the received message L to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) corresponding to the domain name of the IPv4 terminal <b>2</b> from the message L.

The DNS substituting means <b>13</b> forms a message (hereinafter, called a ‘message P’) to notify of an ‘IPv4-mapped IPv6 address’ (: : FFFF : <b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) in which the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> has been set as an IP address corresponding to the domain name of the IPv4 terminal <b>2</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message P to the IPv6 terminal <b>5</b> and sends the message P thereto. The IP transmitting and receiving means <b>10</b> transmits the message P to the IPv6 terminal <b>5</b> via the IPv6 network.

As shown in <figref idref="DRAWINGS">FIG. 9</figref>, the IPv6 terminal <b>5</b> transmits the IPv6 packet in which the IPv6 address (: : FFFF : <b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) notified from the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>has been set into the IPv6 destination address field of the IPv6 packet and the own IPv6 address (<b>1</b> : : <b>1</b>) has been set into the IPv6 source address field to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>through the IPv6 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>sends the received IPv6 packet to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> extracts the IPv6 address (: : FFFF : <b>133</b>. <b>144</b>,. <b>95</b>. <b>101</b>) from the IPv6 destination address field of the IPv6 packet, converts the IPv6 address into the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of 32 bits by deleting the fixed pattern of 96 bits mentioned above, and sets the IPv4 address into the IPv4 destination address field of the IPv4 packet. The IP header converting means <b>11</b> extracts the IPv6 address (<b>1</b> : : <b>1</b>) from the IPv6 source address field of the IPv6 packet and sends it to the IP address conversion information holding means <b>15</b>.

The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> as a key and notifies the IP header converting means <b>11</b> of a result indicating whether the relevant entry has been registered or not.

The IP header converting means <b>11</b> which received the notification that the relevant entry is not registered yet in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> instructs the IPv4 address capturing means <b>14</b> so as to capture the IPv4 address and, after that, waits for a reply for a predetermined time. The IPv4 address capturing means <b>14</b> forms an IPv4 address capturing message (hereinafter, referred to as a “message M”), instructs the IP transmitting and receiving means <b>10</b><i>s </i>as to transfer the message M to the DHCPv4 server <b>4</b>, supplies the message M thereto, and after that, waits for a reply for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message M to the DHCPv4 server <b>4</b> via the IPv4 network.

The DHCPv4 server <b>4</b> which received the message M gives the IPv4 address to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminals <b>5</b> (it is now assumed that <b>133</b>. <b>144</b>. <b>95</b>. <b>1</b> was given) and returns an IPv4 address given message (hereinafter, referred to as a “message N”).

The IP transmitting and receiving means <b>10</b> which received the message N transmits the message N to the IPv4 address capturing means <b>14</b>. The IPv4 address capturing means <b>14</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the message N and notifies the IP header converting means <b>11</b>. The IP header converting means <b>11</b> transmits the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) captured from the DHCPv4 server <b>4</b> to the IP address conversion information holding means <b>15</b> so as to correspond to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers the correspondence between the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) and the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The entry of the IP address conversion table <b>151</b> is deleted when it is not referred to for a predetermined time. The IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) is returned to the DHCPv4 server <b>4</b>.

When a notification that the corresponding entry has been registered in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is received or when the registration of the corresponding entry into the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is completed, the IP header converting means <b>11</b> sets the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) into the IPv4 source address field of the IPv4 packet. The IPv4 packet formed by performing the IP header conversion as mentioned above is sent to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the IPv4 packet to the IPv4 terminal <b>2</b> through the IPv4 network.

In this manner, the communication from the IPv6 terminal <b>5</b> to the IPv4 terminal <b>2</b> can be started.

<figref idref="DRAWINGS">FIGS. 12 and 13</figref> are flowcharts for operation in another case of starting a communication from the IPv4 terminal <b>2</b> to the IPv6 terminal <b>5</b>. This case has a feature in that “IPv4-mapped IPv6 address” is not used.

Although the IPv4 terminal <b>2</b> knows a domain name of the IPv6 terminal <b>5</b>, it doesn't know an IP address. As shown in <figref idref="DRAWINGS">FIG. 12</figref>, therefore, after the IPv4 terminal <b>2</b> transmitted an inquiry message (hereinafter, called a ‘message A’) of the IP address corresponding to the domain name of the IPv6 terminal <b>5</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>via the IPv4 network <b>104</b>, it waits for a response for a predetermined time.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>receives and processes the message A and sends it to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message A to the DNSv4 server <b>3</b>, sends the message A thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message A to the DNSv4 server <b>3</b> via the IPv4 network.

The DNSv4 server <b>3</b> which received the message A retrieves the domain information of the IPv6 terminal <b>5</b>. When it is not registered, however, a response to the message A is not returned.

The DNS substituting means <b>13</b> which didn't receive the response from the DNSv4 server <b>3</b> even after the elapse of a predetermined time instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message A to the DNSv6 server <b>6</b>, sends the message A thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message A to the DNSv6 server <b>6</b> via the IPv6 network.

The DNSv6 server <b>6</b> which received the message A retrieves the domain information of the IPv6 terminal <b>5</b> and returns the registered domain information of the IPv6 terminal <b>5</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>as a response message (hereinafter, referred to as a ‘message B’) for the message A.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>which received the response from the DNSv6 server <b>6</b> within the predetermined time sends the received message B to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> obtains the IPv6 address (<b>1</b> : : <b>1</b>) corresponding to the domain name of the IPv6 terminal <b>5</b> from the message B and sends it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> as a key. If the relevant entry has been registered, the IPv4 address of the entry is returned to the DNS substituting means <b>13</b>. On the other hand, when the relevant entry is not registered yet, such a fact is notified to the DNS substituting means <b>13</b>.

The DNS substituting means <b>13</b> which received the IPv4 address from the Ip address conversion information holding means <b>15</b> forms a message (hereinafter, referred to as a ‘message E’) to notify of the IPv4 address as an IPv4 address corresponding to the domain name of the IPv6 terminal <b>5</b> and sends it to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the message E to the IPv4 terminal <b>2</b> via the IPv4 network.

On the other hand, the DNS substituting means <b>13</b> which received the notification indicating that the entry is not registered yet from the IP address conversion information holding means <b>15</b> instructs the IPv4 address capturing means <b>14</b> so as to capture the IPv4 address and, after that, waits for a response for a predetermined time. The IPv4 address capturing means <b>14</b> forms an IPv4 address capturing message (hereinafter, called a ‘message C’) instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message C to the DHCPv4 server <b>4</b>, sends the message C thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message C to the DHCPv4 server <b>4</b> via the IPv4 network.

The DHCPv4 serve <b>4</b> which received the message C gives the IPv4 address to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> (it is now assumed that “<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>” has been given) and returns an IPv4 address given message (hereinafter, called a ‘message D’).

The IP transmitting and receiving means <b>10</b> which received the message D sends the message D to the IPv4 address capturing means <b>14</b>. The IPv4 address capturing means <b>14</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the message D and notifies the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> forms the message E to notify of the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) as an IP address corresponding to the domain name of the IPv6 terminal <b>5</b> and sends it to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the message E to the IPv4 terminal <b>2</b> via the IPv4 network. The DNS substituting means <b>13</b> sends the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) captured from the DHCPv4 server <b>4</b> to the IP address conversion information holding means <b>15</b> in correspondence to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers a correspondence of the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) and the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The entry of the IP address conversion table <b>151</b> is deleted in the case where it is not referred for a predetermined time. The IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) is returned to the DHCPv4 server <b>4</b>.

As shown in <figref idref="DRAWINGS">FIG. 13</figref>, the IPv4 terminal <b>2</b> transmits the IPv4 packet in which the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) notified from the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>by the message E has been set into an IPv4 destination address field of the IPv4 packet and an own IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) has been set into an IPv4 source address field to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>through the IPv4 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>sends the received IPv4 packet to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> extracts the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) from the IPv4 source address field of the IPv4 packet, and sends it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> as a key. When the relevant entry has been registered, the IPv6 address of the entry is returned to the IP header converting means <b>11</b>. On the other hand, if the relevant entry is not registered yet, such a fact is notified to the IP header converting means <b>11</b>.

The IP header converting means <b>11</b> which received the notification that the entry is not registered from the IP address conversion information holding means <b>15</b> instructs the IPv6 address capturing means <b>20</b> so as to capture the IPv6 address and, after that, waits for a reply for a predetermined time. The IPv6 address capturing means <b>20</b> forms an IPv6 address capturing message (hereinafter, referred to as a “message C<b>6</b>”), instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message C<b>6</b> to the DHCPv6 server <b>7</b> (FIG. <b>11</b>), supplies the message C<b>6</b> thereto, and after that, waits a replay for a predetermined time., The IP transmitting and receiving means <b>10</b> transmits the message C<b>6</b> to the DHCPv6 server <b>7</b> via the IPv6 network.

The DHCPv6 server <b>7</b> which received the message C<b>6</b> gives the IPv6 address to the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> (it is now assumed that <b>1</b> : : <b>101</b> was given) and returns an IPv6 address given message (hereinafter, referred to as a “message D<b>6</b>”).

The IP transmitting and receiving means <b>10</b> which received the message D<b>6</b> supplies the message D<b>6</b> to the IPv6 address capturing means <b>20</b> (FIG. <b>10</b>). The IPv6 address capturing means <b>20</b> obtains an IPv6 address (<b>1</b> : : <b>101</b>) from the message D<b>6</b> and notifies the IP header converting means <b>11</b>.

The IP header converting means <b>11</b> transmits the IPv6 address (<b>1</b> : : <b>101</b>) captured from the DHCPv6 server <b>7</b> to the IP address conversion information holding means <b>15</b> so as to correspond to the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers a correspondence between the IPv6 address (<b>1</b> : : <b>101</b>) and the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b>. The entry in the IP address conversion table <b>151</b> is deleted when it is not referred to for a predetermined time. The IPv6 address (<b>1</b> : : <b>101</b>) is returned to the DHCPv6 server <b>7</b>.

When a notification that the relevant entry has been registered in the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> is received or when the registration of the relevant entry into the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> is completed, the IPv6 address (<b>1</b> : : <b>101</b>) of the relevant entry is set into an IPv6 transmission address field of the IPv6 packet. The IP header converting means <b>11</b> extracts the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the IPv4 destination address field of the IPv4 packet and sends it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> obtains the IPv6 address (<b>1</b> : : <b>1</b>) corresponding to the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) with reference to the IP address conversion table <b>151</b> and sends it to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> sets the IPv6 address (<b>1</b> : : <b>1</b>) into an IPv6 destination address field of the IPv6 packet. The IPv6 packet formed by performing the IP header conversion as mentioned above is sent to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the IPv6 packet to the IPv6 terminal <b>5</b> through the IPv6 network.

In this manner, the communication from the IPv4 terminal <b>2</b> to he IPv6 terminal <b>5</b> can be started.

<figref idref="DRAWINGS">FIGS. 14 and 15</figref> are flowcharts for operation in another case of starting a communication from the IPv6 terminal <b>5</b> to the IPv4 terminal <b>2</b>. This case has the feature in that the “IPv4-mapped IPv6 address” is not used.

Although the IPv6 terminal <b>5</b> knows a domain name of the IPv4 terminal <b>2</b>, it doesn't know the IP address. As shown in <figref idref="DRAWINGS">FIG. 14</figref>, therefore, the IPv6 terminal <b>5</b> transmits an inquiry message (hereinafter, called a ‘message K’) of an IP address corresponding to the domain name of the IPv4 terminal <b>2</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>via the IPv6 network <b>106</b> and, after that, waits for a response for a predetermined time.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>receives and processes the message K and sends it to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message K to the DNSv6 server <b>6</b>, sends the message K thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message K to the DNSv6 server <b>6</b> via the IPv6 network.

The DNSv6 server <b>6</b> which received the message K retrieves the domain information of the IPv4 terminal <b>2</b>. However, when it is not registered, a response to the message K is not returned.

The DNS substituting means <b>13</b> which didn't receive the response from the DNSv6 server <b>6</b> even after the elapse of a predetermined time instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message K to the DNSv4 server <b>3</b>, sends the message K thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message K to the DNSv4 server <b>3</b> via the IPv4 network.

The DNSv4 server <b>3</b> which received the message K retrieves the domain information of the IPv4 terminal <b>2</b> and returns the registered domain information of the IPv4 terminal <b>2</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>as a response message (hereinafter, called a ‘message L’) for the message K.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>which received the response from the DNSv4 server <b>3</b> within the predetermined time sends the received message L to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) corresponding to the domain name of the IPv4 terminal <b>2</b> from the message L.

Subsequently, the DNS substituting means <b>13</b> sends the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> as a key and notifies the DNS substituting means <b>13</b> of information about whether the relevant entry has been registered or not.

The DNS substituting means <b>13</b> which received a notification indicating that the relevant entry is not registered yet in the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> instructs the IPv6 address capturing means <b>20</b> so as to capture the IPv6 address and, after that, waits for a response for a predetermined time. The IPv6 address capturing means <b>20</b> forms an IPv6 address capture message (hereinafter, called a ‘message M<b>6</b>’), instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message M to the DHCPv6 server <b>7</b>, sends the message M thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message M<b>6</b> to the DHCPv6 server <b>7</b> via the IPv6 network.

The DHCPv6 server <b>7</b> which received the message M<b>6</b> gives the IPv6 address to the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> (it is now assumed that “1 : : 101” is given) and returns an IPv6 address given message (hereinafter, called a ‘message N<b>6</b>’).

The IP transmitting and receiving means <b>10</b> which received the message N<b>6</b> sends the message N<b>6</b> to the IPv6 address capturing means <b>20</b>. The IPv6 address capturing means <b>20</b> obtains the IPv6 address (<b>1</b> : : <b>101</b>) from the message N<b>6</b> and notifies the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> sends the IPv6 address (<b>1</b> : : <b>101</b>) captured from the DHCPv6 server <b>7</b> to the IP address conversion information holding means <b>15</b> in correspondence to the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers a correspondence of the IPv6 address (<b>1</b> : : <b>101</b>) and the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b>. The entry in the IP address conversion table <b>151</b> is deleted when it is not referred to for a predetermined time and the IPv6 address (<b>1</b> : : <b>101</b>) is returned to the DHCPv6 server <b>7</b>.

When a notification indicating that the relevant entry has been registered in the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> is received or when the registration of the relevant entry into the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> is completed, the DNS substituting means <b>13</b> forms a message (hereinafter, called a ‘message P’) to notify of the IPv6 address (<b>1</b> : : <b>101</b>) corresponding to the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) of the IPv4 terminal <b>2</b> as an IP address corresponding to the domain name of the IPv4 terminal <b>2</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message P to the IPv6 terminal <b>5</b> and sends the message P thereto. The IP transmitting and receiving means <b>10</b> transmits the message P to the IPv6 terminal <b>5</b> via the IPv6 network.

As shown in <figref idref="DRAWINGS">FIG. 15</figref>, the IPv6 terminal <b>5</b> transmits the IPv6 packet in which the IPv6 address (<b>1</b> : : <b>101</b>) notified from the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>has been set into the IPv6 destination address field of the IPv6 packet and the own IPv6 address (<b>1</b> : : <b>1</b>) has been set into the IPv6 source address field to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>through the IPv6 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>sends the received IPv6 packet to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> extracts the IPv6 address (<b>1</b> : : <b>101</b>) from the IPv6 destination address field of the IPv6 packet, and supplies it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) corresponding to the IPv6 address (<b>1</b> : : <b>101</b>) with reference to the IP address conversion table <b>151</b> and transmits to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> sets the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>101</b>) into the IPv4 destination address field of the IPv4 packet. The IP header converting means <b>11</b> extracts the IPv6 address (<b>1</b> : : <b>1</b>) from the IPv6 source address field of the IPv6 packet and sends it to the IP address conversion information holding means <b>15</b>.

The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> as a key and notifies the IP header converting means <b>11</b> of a result indicating whether the relevant entry has been registered or not.

The IP header converting means <b>11</b> which received the notification that the relevant entry is not registered yet in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> instructs the IPv4 address capturing means <b>14</b> so as to capture the IPv4 address and, after that, waits for a reply for a predetermined time. The IPv4 address capturing means <b>14</b> forms an IPv4 address capturing message (hereinafter, referred to as a “message M”), instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message M to the DHCPv4 server <b>4</b>, supplies the message M thereto, and after that, waits for a reply for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message M to the DHCPv4 server <b>4</b> via the IPv4 network.

The DHCPv4 server <b>4</b> which received the message M gives the IPv4 address to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> (it is now assumed that <b>133</b>. <b>144</b>. <b>95</b>. <b>1</b> was given) and returns an IPv4 address given message (hereinafter, referred to as a “message N”).

The IP transmitting and receiving means <b>10</b> which received the message N transmits the message N to the IPv4 address capturing means <b>14</b>. The IPv4 address capturing means <b>14</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the message N and notifies the IP header converting means <b>11</b>. The IP header converting means <b>11</b> transmits the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) captured from the DHCPv4 server <b>4</b> to the IP address conversion information holding means <b>15</b> so as to correspond to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers the correspondence between the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) and the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The entry of the IP address conversion table <b>151</b> is deleted when it is not referred to for a predetermined time. The IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) is returned to the DHCPv4 server <b>4</b>.

When a notification that the corresponding entry has been registered in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is received or when the registration of the corresponding entry into the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is completed, the IP header converting means <b>11</b> sets the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) into the IPv4 source address field of the IPv4 packet. The IPv4 packet formed by performing the IP header conversion as mentioned above is sent to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the IPv4 packet to the IPv4 terminal <b>2</b> through the IPv4 network.

In this manner, the communication from the IPv6 terminal <b>5</b> to the IPv4 terminal <b>2</b> can be started.

(B) Second Embodiment

<figref idref="DRAWINGS">FIG. 16</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus <b>1</b><i>b </i>according to the second embodiment of the invention.

The IPv4-IPv6 converting apparatus <b>1</b><i>b </i>has a construction obtained by adding IPv4 address pool means <b>16</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>of FIG. <b>1</b>.

At the time of an activation or the like of the IPv4-IPv6 converting apparatus <b>1</b><i>b</i>, the IPv4 address capturing means <b>14</b> captures a plurality of IPv4 addresses in a lump from the DHCPv4 server <b>4</b> and pools them into the IPv4 address pool means <b>16</b>. At the start of the communication between the IPv4 terminal <b>2</b> and IPv6 terminal <b>5</b>, the IPv4 address capturing means <b>14</b> obtains the IPv4 addresses from the IPv4 address pool means <b>16</b>.

By using the IPv4-IPv6 converting apparatus <b>1</b><i>b</i>, a load of the IPv4 network <b>104</b> can be reduced more than the case of capturing the IPv4 addresses one by one from the DHCPv4 server <b>4</b>.

(C) Third Embodiment

<figref idref="DRAWINGS">FIG. 17</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus <b>1</b><i>c </i>according to the third embodiment of the invention.

The IPv4-IPv6 converting apparatus <b>1</b><i>c </i>has a construction obtained by providing DHCPv4 server means <b>17</b> in place of the IPv4 address capturing means <b>14</b> of the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>of FIG. <b>1</b>.

By using the IPv4-IPv6 converting apparatus <b>1</b><i>c</i>, there is no need to individually provide the DHCPv4 server <b>4</b>.

(D) Fourth Embodiment

<figref idref="DRAWINGS">FIG. 18</figref> is a constructional diagram of an IPv4-IPv6 converting apparatus id according to the fourth embodiment of the invention.

The IPv4-IPv6 converting apparatus id has a construction obtained by adding DNSv4 server means <b>18</b> and DNSv6 server means <b>19</b> to the IPv4-IPv6 converting apparatus <b>1</b><i>a </i>of FIG. <b>1</b>.

By using the IPv4-IPv6 converting apparatus <b>1</b><i>d</i>, there is no need to individually provide the DNSv4 server <b>3</b> and DNSv6 server <b>6</b>.

(E) Fifth Embodiment

The fifth embodiment will now be described by using <figref idref="DRAWINGS">FIGS. 19</figref>, <b>20</b>, and <b>21</b>.

<figref idref="DRAWINGS">FIG. 19</figref> is a diagram of a communication network system <b>108</b> in which the IPv6 network <b>106</b> and an IPv6 network <b>107</b> are connected through the IPv4 network <b>104</b>. The communication network system <b>108</b> has a construction obtained by adding the IPv6 network <b>107</b> to the communication network system <b>100</b> of FIG. <b>3</b>. The IPv6 network <b>107</b> is connected to the IPv4 network <b>104</b> by an IPv4-IPv6 converting apparatus <b>111</b>.

In addition to an IPv6 terminal <b>115</b>, a DNSv6 server <b>116</b> for managing domain information in the IPv6 network <b>107</b> is connected to the IPv6 network <b>107</b>.

It is now assumed that (<b>2</b> : : <b>1</b>) has been allocated as an IPv6 address to the IPv6 terminal <b>115</b>. In addition to 100 IPv4 addresses of “<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>” to “<b>133</b>. <b>144</b>. <b>95</b>. <b>100</b>”, 100 IPv4 addresses of “<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>” to “<b>133</b>. <b>144</b>. <b>96</b>. <b>100</b>” have been pooled in the DHCPv4 server <b>4</b>. It is also assumed that the IPv4-IPv6 converting apparatus <b>111</b> has been set as a DNS server in the IPv6 terminal <b>115</b>. It is assumed that the DNSv6 server <b>116</b> has been set as a DNS server and the DHCPv4 server <b>4</b> has been set as a DHCP server into the IPv4-IPv6 converting apparatus <b>111</b>. It is also assumed that the IPv4-IPv6 converting apparatus <b>111</b> has been set as a DNS server of an upper domain into an IPv4-IPv6 converting apparatus <b>1</b>.

<figref idref="DRAWINGS">FIGS. 20 and 21</figref> are flowcharts for the operation in case of starting a communication from the IPv6 terminal <b>5</b> to the IPv6 terminal <b>115</b>.

Although the IPv6 terminal <b>5</b> knows a domain name of the IPv6 terminal <b>115</b>, it doesn't know an IP address. Therefore, as shown in <figref idref="DRAWINGS">FIG. 20</figref>, the IPv6 terminal <b>5</b> transmits an inquiry message (hereinafter, called a ‘message Q’) of the IP address corresponding to the domain name of the IPv6 terminal <b>115</b> to the IPv4-IPv6 converting apparatus <b>1</b> via the IPv6 network <b>106</b> and, after that, waits for a response for a predetermined time.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b> receives and processes the message Q and sends it to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message Q to the IPv4-IPv6 converting apparatus <b>111</b>, sends the message Q thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message Q to the IPv4-IPv6 converting apparatus <b>111</b> via the IPv4 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>111</b> receives and processes the message Q and sends it to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message Q to the DNSv6 server <b>116</b>, sends the message Q thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transfers the message Q to the DNSv6 server <b>116</b> via the IPv6 network.

The DNSv6 server <b>116</b> which received the message Q retrieves domain information of the IPv6 terminal <b>115</b> and returns the registered domain information of the IPv6 terminal <b>115</b> to the IPv4-IPv6 converting apparatus <b>111</b> as a response message (hereinafter, called a ‘message R’) for the message Q.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>111</b> which received the response from the DNSv6 server <b>116</b> within a predetermined time sends the received message R to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> obtains an IPv6 address (<b>2</b> : : <b>1</b>) corresponding to the domain name of the IPv6 terminal <b>115</b> from the message R and sends it to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv6 address (<b>2</b> : : <b>1</b>) of the IPv6 terminal <b>115</b> as a key. If the relevant entry has been registered, the holding means <b>15</b> returns the IPv4 address of the entry to the DNS substituting means <b>13</b>. On the other hand, if the relevant entry is not registered yet, such a fact is notified to the DNS substituting means <b>13</b>.

The DNS substituting means <b>13</b> which received the IPv4 address from the IP address conversion information holding means <b>15</b> forms a message (hereinafter, called a ‘message U’) to notify of the IPv4 address as an IPv4 address corresponding to the domain name of the IPv6 terminal <b>115</b> and sends the message U to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the message U to the IPv4-IPv6 converting apparatus <b>1</b> via the IPv4 network.

The DNS substituting means <b>13</b> which received the notification that the entry is not registered yet from the IP address conversion information holding means <b>15</b> instructs the IPv4 address capturing means <b>14</b> so as to capture the IPv4 address and, after that, waits for a response for a predetermined time. The IPv4 address capturing means <b>14</b> forms an IPv4 address capturing message (hereinafter, called a ‘message S’), instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message S to the DHCPv4 server <b>4</b>, sends the message S thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message S to the DHCPv4 server <b>4</b> via the IPv4 network.

The DHCPv4 server <b>4</b> which received the message S gives the IPv4 address to the IPv6 address (<b>2</b> : : <b>1</b>) of the IPv6 terminal <b>115</b> (it is now assumed that “<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>” is given) and returns an IPv4 address given message (hereinafter, called a ‘message T’).

The IP transmitting and receiving means <b>10</b> which received the message T sends the message T to the IPv4 address capturing means <b>14</b>. The IPv4 address capturing means <b>14</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) from the message T and notifies the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> forms the message U to notify of the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) as an IP address corresponding to the domain name of the IPv6 terminal <b>115</b> and sends it to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the message U to the IPv4-IPv6 converting apparatus <b>1</b> via the IPv4 network. The DNS substituting means <b>13</b> sends the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) captured from the DHCPv4 server <b>4</b> to the IP address conversion information holding means <b>15</b> in correspondence to the IPv6 address (<b>2</b> : : <b>1</b>) of the IPv6 terminal <b>115</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers a correspondence of the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) and the IPv6 address (<b>2</b> : : <b>1</b>) of the IPv6 terminal <b>115</b>. The entry in the IP address conversion table <b>151</b> is deleted when it is not referred to for a predetermined time. The IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) is returned to the DHCPv4 server <b>4</b>.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b> which received the response from the IPv4-IPv6 converting apparatus <b>111</b> within the predetermined time sends the received message U to the DNS substituting means <b>13</b>. The DNS substituting means <b>13</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) corresponding to the domain name of the IPv6 terminal <b>115</b> from the message U. The DNS substituting means <b>13</b> forms a message (hereinafter, called a ‘message V’) to notify of an ‘IPv4-mapped IPv6 address’ (: : FFFF : <b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) in which the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) derived from the message U has been set as an IP address corresponding to the domain name of the IPv6 terminal <b>115</b>, instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message V to the IPv6 terminal <b>5</b>, and sends the message V thereto. The IP transmitting and receiving means <b>10</b> transmits the message V to the IPv6 terminal <b>5</b> via the IPv6 network.

As shown in <figref idref="DRAWINGS">FIG. 21</figref>, the IPv6 terminal <b>5</b> transmits the IPv6 packet in which the IPv6 address (: FFFF : <b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) notified from the IPv4-IPv6 converting apparatus <b>1</b> has been set into the IPv6 destination address field of the IPv6 packet and the own IPv6 address (<b>1</b> : : <b>1</b>) has been set into the IPv6 source address field to the IPv4-IPv6 converting apparatus <b>1</b> through the IPv6 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>1</b> sends the received IPv6 packet to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> extracts the IPv6 address (: : FFFF : <b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) from the IPv6 destination address field of the IPv6 packet, converts the IPv6 address to the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) of 32 bits by deleting the fixed pattern of 96 bits mentioned above, and sets the IPv4 address into the IPv4 destination address field of the IPv4 packet. The IP header converting means <b>11</b> extracts the IPv6 address (<b>1</b> : : <b>1</b>) from the IPv6 source address field of the IPv6 packet and sends it to the IP address conversion information holding means <b>15</b>.

The IP address conversion information holding means <b>15</b> retrieves the IP address conversion table <b>151</b> by using the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> as a key and notifies the IP header converting means <b>11</b> of a message indicating whether the relevant entry has been registered or not.

The IP header converting means <b>11</b> which received the notification that the relevant entry is not registered yet in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> instructs the IPv4 address capturing means <b>14</b> so as to capture the IPv4 address and, after that, waits for a response for a predetermined time. The IPv4 address capturing means <b>14</b> forms an IPv4 address capturing message (hereinafter, called a ‘message W’), instructs the IP transmitting and receiving means <b>10</b> so as to transfer the message W to the DHCPv4 server <b>4</b>, sends the message W thereto, and after that, waits for a response for a predetermined time. The IP transmitting and receiving means <b>10</b> transmits the message W to the DHCPv4 server <b>4</b> via the IPv4 network.

The DHCPv4 server <b>4</b> which received the message W gives the IPv4 address to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> (it is now assumed that “<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>” is given) and returns an IPv4 address given message (hereinafter, called a ‘message X’).

The IP transmitting and receiving means <b>10</b> which received the message X sends the message X to the IPv4 address capturing means <b>14</b>. The IPv4 address capturing means <b>14</b> obtains the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the message X and notifies the IP header converting means <b>11</b>. The IP header converting means <b>11</b> sends the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) captured from the DHCPv4 server <b>4</b> to the IP address conversion information holding means <b>15</b> in correspondence to the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The IP address conversion information holding means <b>15</b> forms a new entry into the IP address conversion table <b>151</b> and registers a correspondence of the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) and the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b>. The entry in the IP address conversion table <b>151</b> is deleted when it is not referred to for a predetermined time. The IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) is returned to the DHCPv4 server <b>4</b>.

When a notification that the relevant entry has been registered in the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is received or when the registration of the relevant entry into the IPv6 address (<b>1</b> : : <b>1</b>) of the IPv6 terminal <b>5</b> is completed, the IP header converting means <b>11</b> sets the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) into the IPv4 source address field of the IPv4 packet. The IPv4 packet formed by performing the IP header conversion as mentioned above is sent to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the IPv4 packet to the IPv4-IPv6 converting apparatus <b>111</b> through the IPv4 network.

The IP transmitting and receiving means <b>10</b> of the IPv4-IPv6 converting apparatus <b>111</b> sends the received IPv4 packet to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> extracts the IPv4 address (<b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) from the IPv4 source address field of the IPv4 packet, converts the IPv4 address to an ‘IPv4-mapped IPv6 address’ (: : FFFF : <b>133</b>. <b>144</b>. <b>95</b>. <b>1</b>) of 128 bits by adding the fixed pattern of 96 bits mentioned above, and sets it into the IPv6 source address field of the IPv6 packet. The IP header converting means <b>11</b> extracts the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) from the IPv4 destination address field of the IPv4 packet and sends the IPv4 address to the IP address conversion information holding means <b>15</b>. The IP address conversion information holding means <b>15</b> obtains the IPv6 address (<b>2</b> : : <b>1</b>) corresponding to the IPv4 address (<b>133</b>. <b>144</b>. <b>96</b>. <b>1</b>) with reference to the IP address conversion table <b>151</b> and sends the IPv6 address to the IP header converting means <b>11</b>. The IP header converting means <b>11</b> sets the IPv6 address (<b>2</b> : : <b>1</b>) into the IPv6 destination address field of the IPv6 packet. The IPv6 packet formed by performing the IP header conversion as mentioned above is sent to the IP transmitting and receiving means <b>10</b>. The IP transmitting and receiving means <b>10</b> transmits the IPv6 packet to the IPv6 terminal <b>115</b> through the IPv6 network.

In this manner, the communication from the IPv6 terminal <b>5</b> to the IPv6 terminal <b>115</b> can be started.

Although the invention has been described above by using the communication between the two IPv6 terminals when there is the IPv4 network on the communication path, a similar construction can be also applied to a communication between the two IPv4 terminals when there is the IPv6 network on the communication path.

Although the invention has been also described above by using the communication between the two IPv6 terminals when there is only one IPv4 network on the communication path, a similar construction can be also applied to a communication between the two IPv6 terminals when there are a plurality of IPv4 networks and IPv6 networks on the communication path, a communication between the two IPv4 terminals, and a communication between the IPv4 terminal and the IPv6 terminal.

According to the IPv4-IPv6 communicating method and IPv4-IPv6 converting apparatus of the invention, only the IPv6 protocol is installed to the IPv6 terminal and even if the IPv4 address is not previously fixedly allocated, the communication between the IPv4 terminal and the IPv6 terminal can be executed. Further, since the partner can be designated not by the actual IP address but by the domain name, the communication can be performed without mutually being aware whether the partner uses the IPv4 or IPv6 as a communication protocol.

As for the registration and deletion of the entry in the conversion table of the IPv4 address and IPv6 address, by limiting by the setting of a manager, the system can also become a fire wall between the IPv4 network and the IPv6 network.

A communication between the two IPv6 terminals partitioned by the IPv4 network can be also executed without preliminarily performing any special setting. Similarly, a communication between the two IPv4 terminals partitioned by the IPv6 network can be executed without preliminarily performing any special setting.

Further, even if any special setting is not preliminarily performed, a communication between the IPv4 terminals partitioned by a plurality of IPv4 networks and IPv6 networks, a communication between the IPv6 terminals, and a communication between the IPv4 terminal and the IPv6 terminal can be executed.